Many types of protective coatings are routinely applied to industrial and consumer goods and materials. Examples include coil coatings for metals, wood coatings for furniture, flooring and joinery and highly transparent glossy topcoats for automobiles. Such coatings are used to protect the underlying substrate from damage caused by impacts, scratching, moisture, light and other environmental factors to which the surface is exposed. The coatings are hard in order to resist scratching and impact damage and they commonly consist of highly crosslinked thermoset polymers. Traditionally, hardness is achieved by using a polymer possessing a stiff polymer chain and a high degree of cross-linking. However, the incorporation of this type of polymer may lead to coatings which are brittle and prone to cracking and flaking. Recent interest has focussed on the incorporation of inorganic particles possessing a median diameter of less than about 50 nm in order to improve the scratch and abrasion resistance of coatings. It is generally considered that such particles need to possess median diameters of 100 nm or less in order to impart the required properties and retain acceptable levels of transparency.
Mineral fillers are frequently added to opaque and pigmented coatings but not to transparent protective coatings. In opaque and pigmented coatings, their role is to increase the opacifying efficiency of TiO2 pigment particles by spacing them out, and, in some cases, to help opacify the coating by generating light scattering of their own.
It is also known to combine mineral fillers with polymers to modify the mechanical properties. However, retaining transparency is traditionally not of importance for these applications and therefore the effect of the filler on the transparency is of minimal concern. More recently, the use of nanoclay fillers in polymers has received attention primarily because it has been shown to be possible to achieve comparable stiffness at significantly lower addition levels than with conventional micron sized mineral filler particles. However, these nanoclay fillers are based on montmorillonite clays which are a type of swelling clay mineral. Montmorillonite clays require complete delamination, exfoliation and surface coating before use and must remain fully dispersed in the polymer in order to deliver these benefits. The requirement for complete surface coating makes nanoclays generally more expensive than the polymers in which they are used and adequate dispersion is rarely achieved using conventional mixing and compounding equipment.
There is a continued need for clear coatings, which may also be referred to as clear film forming compositions, which possess good mechanical properties. However, in providing acceptable (or preferably improved) mechanical properties then optical properties such as the degree of transparency should not be compromised to a significant degree such, for example, that the composition is not substantially transparent.
The present invention is based, inter alia, on the surprising finding that a range of particulate inorganic minerals, for example, non-swelling clay minerals such as kandite clays, talc and other minerals such as mica, silica, perlite and diatomaceous earth (DE) may be used in clear coatings and provide a combination of good mechanical and optical properties.